On the baked top of a wall or the shoulder of an old gravestone, where most mosses dare not grow, you find tight grey domes that look as though they have been dusted with frost. That hoary cushion is common pincushion, a moss that has made its living in the one place the soft green carpets cannot follow: bare stone in full sun.
Recognising it
Look first for a neat, rounded cushion, seldom wider than a coin, packed tight enough to shed water. Its colour is a dull grey-green, and the grey is the point: each leaf ends in a long, clear, glassy hair, and the massed hair-points frost the whole dome so it reads silver from a pace away. Run a dry fingertip over it and it feels harsh and bristly. The cushions sit so firmly to the stone that you can rock one with a fingernail and feel it resist, anchored by a felt of rhizoids into every pore of the surface.
The capsules that hide
Catch it in fruit and naming it becomes almost certain. Where most mosses hoist their spore capsules proudly clear of the leaves on straight stalks, Grimmia pulvinata does the reverse: its stalk curves over like a shepherd's crook so that the ripening capsule is drawn back down and tucked into the cushion, half buried among the hairs. Part the hair-points and the little egg-shaped capsules lie there on their arched stalks, a quirk so reliable that it alone settles the identification. As the capsule dries and the spores ripen the stalk slowly straightens and lifts it clear again to scatter.
A moss for sun and stone
This is a plant of hard, sunlit surfaces. It colonises the mortared tops of walls, concrete, asphalt-capped posts, tombstones, limestone and other base-rich rock, generally where there is a little lime and plenty of light, conditions that would scorch a woodland moss to dust. It belongs to the small band of urban mosses that thrive on the built environment, keeping company with the wall screw-moss of mortar joints and the silvery thread-moss of pavement cracks. In town it is genuinely everywhere, though so dry and grey that most people walk past it without a glance.
How it survives the drought
Living on sun-baked stone means coping with long spells without water, and the cushion is built for exactly that. Drawn together into a dome, the shoots shade one another and cut the surface left open to drying wind. The glassy hair-points work two ways at once, scattering fierce sunlight away from the living tissue below and snagging dew and mist so that droplets run down into the cushion. When the last of the water has gone the plant simply shuts down, greying and brittle, then revives within minutes of a shower, a feat of desiccation tolerance common to the mosses and explored in reviving dried-out moss.
Telling it apart, and leaving it be
The moss most often confused with it is wall screw-moss, which shares the wall top and the hair-points but grows greener, in looser tufts, and stands its capsules upright on straight stalks rather than burying them. The buried, arch-stalked capsule settles the question every time. As a tight, upright, dome-forming moss the common pincushion sits among the acrocarps, the cushion-builders contrasted with the creepers in acrocarpous and pleurocarpous mosses. There is little point trying to cultivate it, since it wants a baking exposure no terrarium or garden bed can offer, but it rewards a closer look on any walk through a churchyard, where a hand lens turns those frosted grey domes into one of the small marvels of the masonry.